1. Field of the Invention
The present invention relates to a carburization and quenching apparatus and a method of quenching thin plate parts, and in particular to a carburization and quenching apparatus for carburizing and quenching thin plate parts and a method of quenching the same.
2. Description of the Background Art
As a bearing ring of a thrust needle bearing, an annular, thin plate part is employed which is formed of chrome molybdenum steel and has a thickness of approximately 1 mm and a diameter of approximately 60 mm. The thin plate part requires some hardness to provide the function as e.g. a bearing ring. In order to obtain the hardness to provide this function, the thin plate part need be carburized and then quenched.
The carburization and quenching process has conventionally employed continuous furnace of mesh belt type, all-casing furnace and the like. The continuous furnace of mesh belt type requires a wide and long mesh belt and thus disadvantageously requires large space. The all-casing furnace disadvantageously has a limited process capability, since it is not until one batch in the carburizing chamber is completely carburized that the next batch enters the carburizing chamber.
For the conventional continuous furnace of mesh belt type, all-casing furnace and the like, a liquid such as oil is employed for quenching carburized thin plate parts to obtain the hardness depending on the application.
However, quenching thin plate parts with a liquid such as oil will result in an excessive cooling rate of the thin plate parts. The excessive cooling rate, together with the extremely reduced thickness of the thin plate parts, will cause thermal treatment-caused distortion (i.e. warpage) in the thin plate parts. Particularly when thermal treatment-caused distortion is caused in the outer ring of a needle bearing, the bearing ring of a thrust bearing or the like, the rolling element hardly operates normally, resulting in degradation of the performance of the bearing. Thus, distortion removal is required to remove such distortion.
Conventionally, thin plate parts are dipped into and thus cooled in a liquid such as oil, and the liquid will thus adhere to the surfaces of the thin plate parts. This also disadvantageously entails the necessity of the washing step for removing the liquid from the thin plate parts.
Furthermore, conventional gas-cooled furnaces which cool carburized thin plate parts with gas can only provide insufficiently quenched and hardened thin plate parts owing to the dropped temperature of the products, since conventional gas-cooling furnaces require some time to move the tray which carries completely carburized thin plate parts and also require some time to raise the pressure of the cooling gas in the cooling chamber.
For batch furnace, such as conventional all-casing furnaces, it is general that the ambient of carburization in a furnace is adjusted after the temperature of products is raised and the soaking process is completed. Thus, products will be inserted into a furnace which contains a gas for carburization used in the immediately preceding process. As a result, the products will be carburized while the temperature of the products is raised, and thus it has been difficult to provide a thin carburization layer in a thin plate part, such as a bearing ring of a thrust needle bearing.